Synchronous motors, including permanent magnet motors and line-start permanent magnet motors, are typically very efficient. However, synchronous motors have limited capability to pull into synchronism loads that have a high torque or high inertia. Accordingly, the benefits in efficiency gains and energy savings ordinarily associated with synchronous motors are not typically achieved in applications having loads with high inertia and/or high torque characteristics. In the past, an inverter has been used with synchronous motors in such applications to power the motor during starting. However, an inverter adds substantial costs and degrades system efficiency.
To achieve the benefits of efficiency provided by synchronous motors, for instance, permanent magnet line-start motors, a soft-start element may be incorporated in the construction of the motor, specifically, between the rotor and the shaft. The soft-start element may be formed integrally within the motor housing and used as an interface between the synchronous motor rotor and the synchronous motor shaft. The soft start element of the motor output shaft may enable using the synchronous motor in applications involving a high inertia and/or high torque load. With the soft-start element, the synchronous motor may be able to bring such a load up to or near synchronous speed. The integrally formed soft-start element effectively isolates the synchronous motor from the high inertia and/or high torque load for enough time to enable the synchronous motor to come up to full synchronous speed. The integrally formed soft-start element then brings the load up to or near synchronous speed. In the case of a locking-type of soft-start element, the load speed will eventually be the same as the speed of the synchronous motor. For the case of a soft-start element that continues to slip even at steady state, the load speed will only approach but not equal the motor speed. As described below, a synchronous motor may be started directly across a polyphase power system and provide a dramatic reduction in noise and mechanical stress during the starting process due to a great reduction in torque ripple transmitted to the driven load. The soft-start element enables rapid acceleration of the motor by isolating the load from the motor temporarily until the motor comes up to synchronous speed.